This is a photo of the nut, it's a hexagonal nut and I have an Allan key set but don't know which one to use to loosen this.
It's really hard to judge scale from a photo.
$£€ 12 will buy you two sets of allen keys - metric & imperial. they'll last you a lifetime. The upside with allen keys is they only come in quite a limited range of sizes, so sets are reasonably small.
But you do need both sets to be sure. It's not good with allen keys to try use one that almost fits, because if you round it out, then you up the ante into bolt extractors; much more skill required.